Modulation of cellular signaling mechanisms by qualitative differences in dietary proteins and their metabolites were studied at the level of: A. Iminodipeptides B. Release of pyrroline 5-carboxylate (P5C) C. Effect of pyrroline 5-carboxylate on mitogenesis. D. Effect of pyrroline 5-carboxylate in prostate cancer cells A. Iminodipeptides. Dipeptides containing proline or hydroxyproline circulate in plasma and are delivered to tissues where they are hydrolyzed by prolidase. Using a proline-auxotrophic cell line, we showed that a variety of iminodipeptides could satisfy the proline requirements for growth. This is the first demonstration that iminodipeptides can serve as the sole source of proline. B. Release of pyrroline 5-carboxylate. Previous work showed that P5C circulates in human plasma. In cultured human fibroblasts, P5C is released into the medium. The rate of accumulation is markedly increased by serum, insulin, and insulin-like growth factor-1 but not by platelet-derived growth factor. Thus, the production and/or release of P5C is under hormone control. C. Effect of pyrroline 5-carboxylate on mitogenesis. P5C stimulates PRPP and purine ribonucleotide synthesis synergistically with platelet-derived growth factor. It also increases the incorporation of thymidine in serum-activated cells. Inhibitor studies suggest that the effect is due to the production of inositol phosphates. D. Effect of pyrroline 5-carboxylate in prostate cancer cells. Since the conversion of testosterone to dihydrotestosterone is dependent on NADPH as a cofactor, we studied the effects of P5C on prostate cancer cells. We found that there is a decreased P5Cstimulated flux in androgen sensitive cells. There is a different level of P5C reductase mRNA in androgen sensitive vs. insensitive cells.